Methods and apparatus of this kind are known from German Pat. No. 20 57 594 and from U.S. Pat. No. 3,724,471. These mixers for gluing wood chips have revolutionized chip gluing, because an extremely fine distribution of the glue on the surface of the chips took place in such mixers in a minimum of space and at a maximum intensity. This produced a significant saving of glue as well as of plant costs in comparison with previously conventional, high-volume axial or centrifugal and helical mixers. The mechanism of operation in these so-called ring mixers is such that the material to be mixed is accelerated by a mixer driven far faster than critical speed such that the material moves in a helical pattern, in the form of a relatively thin ring of material, on the inner wall of the mixing vessel. The glue is introduced directly into this ring of material. In the forms of embodiment described in the documents cited, the glue is introduced through the hollow mixing shaft and through liquid feed pipes radially extending from the shaft and dipping into the ring of material to be mixed. In another form of embodiment, described in German Pat. No. 21 34 305, the glue is introduced through liquid feed pipes which are fixed in the vessel wall in a locally stationary manner and pass through the vessel wall into the interior of the mixing vessel; the outlet openings of these liquid feed pipes terminate in the ring of material to be mixed.
Despite the substantial advantages attained in terms of structure, apparatus and engineering methods by the gluing mixer described above, the disadvantage remained that a large proportion of the wood chips was damaged by the extremely large acceleration forces, which caused an undesirable reduction in the quality of the particle boards made from such chips.
This was true particularly in the case of inferior woods.